In thermostatic control of heating and cooling of a space, a setback feature provides a modest amount of energy savings at almost no cost in the design of the thermostat. By setback is meant the capability to change the setpoint of the thermostat to an energy saving value at certain times of the day or week. For example, if a space such as a dwelling is unoccupied during the day, energy can be saved by reducing the thermostat setpoint temperature during the winter and perhaps raising it during the summer when air conditioning is being used. In a commercial space, the temperature setpoint can be changed to an energy-saving level when the store is closed at night and on weekends. The amount of energy saving is dependent on the difference between the setback and the normal setpoint values. Particularly when there is a substantial difference between the setback and normal setpoint values, it is difficult to predict when recovery from the setback value to the normal value should begin to assure that the occupants of the space will not be uncomfortable because recovery has not been completed at the scheduled time. To avoid this discomfort, it is typical for the occupants to start recovery in ample time to assure that recovery will be completed when desired.
The recovery time depends on the setback temperature, thermal load, heating plant characteristics, and the new setpoint temperature. One can see that determining the recovery time for a particular situation is complex and at best only a prediction because thermal load can unexpectedly vary even during the recovery period. Because of these problems, it is difficult to accurately predict the recovery time required.
There are a number of thermostat designs which attempt to improve the accuracy of determining recovery time. For example, U.S. Pat. Nos. 4,660,759 (Barnard et al.); 4,706,882 (Barnard); 4,522,336 (Culp); and Japanese Appl. No. 54-75977 (Kamimura) all teach calculating a recovery time based on the length of previous recovery times. That is, the thermostat functions according to an algorithm which records previous recovery times and adjusts the next recovery time based on the current space temperature and the new or normal setpoint temperature.
The Barnard and Barnard et al. patents as well as U.S. Pat. No. 4,390,959 (Cabou) include the outside air temperature as an additional variable on which the recovery time calculation is based. These approaches improve the performance of setback thermostats. However, reliance on previous recovery times does not take into account the non-linearities inherent in the response of space temperature to the independent variables affecting it. While performance is improved by using outside temperature to adjust response, this requires an outside temperature sensor which is an additional expense and complexity, and sometimes unsuitable for smaller installations.